1. Field of the Invention
The present invention relates to a liquid crystal display element, and in other words, a liquid crystal light modulation element and method of producing the same.
2. Description of the Background Art
The liquid crystal display element and, in other words, the liquid crystal light modulation element primarily includes a pair of substrates, between which a liquid crystal layer including liquid crystal material is held. For example, predetermined drive voltage is applied to the liquid crystal layer to control orientation of liquid crystal molecules in the liquid crystal layer so that external light incident on the liquid crystal light modulation element is modulated to perform intended display of images or the like.
The liquid crystal light modulation element using the cholesteric liquid crystal has been known as the above kind of liquid crystal light modulation element, and various studies have been made.
Examples of the cholesteric liquid crystal are, e.g., liquid crystal, which exhibits the cholesteric phase by itself, and chiral nematic liquid crystal obtained by adding a chiral agent to nematic liquid crystal.
The cholesteric liquid crystal has such a feature that the liquid crystal molecules form helical structures, and can exhibit three states, i.e., a planar state, focal conic state and a homeotropic state when it is held between a pair of substrates, and is subjected to an external stimulus such as an electric field, a magnetic field or a heat.
In the liquid crystal light modulation element (e.g., liquid crystal display element) using the cholesteric liquid crystal, these three states exhibit different light transparencies and reflectances. Therefore, the three states and the manner of applying the external stimulus can be appropriately selected to perform the display. For example, the display may be performed in the cholesteric-nematic phase transfer mode using the homeotropic state and the focal conic state, and may be performed in a bistable mode using the planar state and the focal conic state.
Among them, the display in the bistable mode has such a feature that the planar state and the focal conic state are stable even in the state where no external stimulus is applied, and thus has the bistability (memory property), which maintains the display state even when no external stimulus (e.g., voltage) is applied. For the above reason, the liquid crystal light modulation element using the cholesteric liquid crystal has been increasingly studied in recent years as the memorizable element (display element achieving the stable display state).
In particular, the liquid crystal light modulation element using the cholesteric liquid crystal, which exhibits the selective reflection property in the visible wavelength range when it is in the planar state, has the memorizable property, and can achieve a bright reflection state. In other words, it can perform bright display without using a polarizing plate or a color filter. Therefore, it is expected that the liquid crystal light modulation element described above can be used as a display element, which is very effective at reducing the power consumption, and can be used as a display element of, e.g., a mobile telephone requiring low power consumption.
The liquid crystal having the bistability can be stable in both the planar state (i.e., the state of the planar orientation), where the helical axis of the cholesteric liquid crystal is substantially perpendicular to the substrate surface, and the liquid crystal exhibits the selective reflection state, and the focal conic state (the state of the focal conic orientation), where the helical axis of the liquid crystal is substantially parallel to the substrate surface, and the liquid crystal is transparent to the visible light.
However, in the liquid crystal display element utilizing the selective reflection characteristics of the cholesteric liquid crystal, the reflection wavelength shifts toward the shorter side in accordance with the incident angle of the light and observation angle because it employs the reflection manner using the light interference.
This phenomenon becomes more remarkable as the helical axis of the cholesteric liquid crystal in the planar orientation is closer to the vertical direction to the substrate surface. In particular, a TN liquid crystal element and an STN liquid crystal element may use a pair of substrates having deposited and rubbed polyimide thin films thereon for holding a liquid crystal layer therebetween, in which case the helical axis of the cholesteric liquid crystal is perfectly or substantially perfectly perpendicular to the substrate surface, resulting in an extremely narrow view angle. If the above liquid crystal element is used as the display element, therefore, the viewability becomes extremely low.
The rubbing of the thin polyimide film increases the restricting force on a polyimide interface so that it becomes difficult to maintain the focal conic state. Consequently, the bistability, which is the distinctive feature of the cholesteric liquid crystal, may be lost.
For avoiding the above, it has been attempted to incline slightly the helical axis of the cholesteric liquid crystal with respect to the normal of the substrate. One of such attempts is called PSCT (Polymer Stabilized Cholesteric Texture), in which polymers are dispersed in the cholesteric liquid crystal so that the helical axes may be positioned in random directions owing to mutual operations between the polymers and the liquid crystal (U.S. Pat. No. 5,384,067). According to this method, however, mixing of the polymer in the liquid crystal material may lower the reliability of the element, and/or may require the increased drive voltage.
In another method, a polyimide film not subjected to the rubbing is deposited on substrate surface opposed to the liquid crystal so that the helical axis may be inclined. In this method, however, domains including different directions of the inclined helical axes (directions of the helical axes projected onto the substrate) are formed randomly so that scattering of the incident light is liable to occur due to the difference in refractive index between the domains, resulting in lowering of the purity of the display color in the selective reflection. In a multilayer liquid crystal display element employing a multilayer structure for multicolor display, the reflection light from the lower layer is liable to be affected by light scattering by an upper layer, which lowers both the contrast and color purity.
For improving the characteristics of the cholesteric liquid crystal element, in which the liquid crystal is held between the substrates provided with the polyimide films not subjected to the orientation processing, Japanese Laid-open Patent Publication No. 10-31205 (31205/1998) has disclosed the following manner. Different surface treatments are effected on the polyimide films formed on the substrates on the observation side and the non-observation (opposite) side, respectively. More specifically, the rubbing processing is effected on only the polyimide film on the non-observation side, and the liquid crystal domains on the observation side may be the non-orientation random domains (polydomain state). Thereby, the helical axes of the liquid crystal on the non-observation side may be substantially perfectly perpendicular to the substrate surface, and the liquid crystal domains on the non-observation side may be uniform (mono-domain state).
According to this manner, however, the rubbing is effected on the whole polyimide film area of the substrate on the non-observation side. Therefore, the liquid crystal domains form the monodomain state on the whole substrate so that the stability in the focal conic state is liable to lower, and the bistability, which is the feature of the cholesteric liquid crystal element, is may be impaired. In the planar orientation state, the inclination of helical axes of the liquid crystal on the random domain side is gradually lost, which impairs the long-term bistability. In any one of the above case, it is difficult to maintain the display state (good display state with high contrast and color purity) for a long time without voltage application, and it is difficult to achieve the intended characteristics for high contrast and high color purity together with the bistability.
In the focal conic state of the cholesteric liquid crystal, the helical axes of liquid crystal molecules are parallel to the substrate plane. Usually, the liquid crystal has a plurality of liquid crystal molecule regions (liquid crystal domains). In the focal conic state, the helical axes of the liquid crystal are parallel or substantially parallel to each other in each liquid crystal domain, but the directions F′ of the helical axes in the neighboring liquid crystal domains are not parallel to each other as shown in FIG. 29. Accordingly, due to the difference in refractive index between the liquid crystal domains, the light incident on the liquid crystal element is slightly scattered at an interface between the liquid crystal domains. In particular, if the helical pitch is small (more specifically, if the helical pitch of the liquid crystal in the planar state is small to cause the selective reflection in the visible range), the liquid crystal domains become small in principle, and the light scattering occurs to a large extent in the element so that employment thereof in the display element cause low contrast.
It is also known to use an element (multilayer liquid crystal element) formed of a plurality of liquid crystal layers stacked together and, e.g., having different selective reflection wavelengths, respectively, for providing a multilayer liquid crystal light modulation element, which allows color display in two or more colors (e.g., full color display). In the case of this multilayer structure, multiple-scattering or the like between the liquid crystal layers particularly increases the influence due to the scattering between the domains so that the contrast is liable to be low.
In the display region of the liquid crystal display element (liquid crystal light modulation element), electrodes are not located on the opposite sides of the liquid crystal in the region other than the pixels, and thus, the non-pixel region (the inter-pixel region). Therefore, the molecules of the liquid crystal in such region cannot be controlled. This results in the following disadvantage.
If the liquid crystal between the substrates is in the planar state (e.g., in the case where a multilayer liquid crystal display element is to be formed by stacking and adhering the plurality of liquid crystal display elements under a pressure, and particularly the liquid crystal between the substrates in each liquid crystal display element is in the planar state due to the pressure), a predetermined voltage may be applied to the liquid crystal of the pixel(s) in one or more liquid crystal display elements for changing the liquid crystal in the pixel(s) into the focal conic state, whereby the molecular orientation of the liquid crystal of the pixel(s) is controlled to attain the focal conic state, as shown in FIG. 5. However, the liquid crystal between the neighboring pixels is affected by the applied voltage, and thereby partially attains the focal conic state so that the focal conic state and the planar state are mixed in the liquid crystal between the pixels. In this mixed state, the domains of the different state may be adjacent to each other. In general, as compared with the case of only the planar state alone, the domains are small in the case where the two states are mixed, and therefore incident light is liable to scatter. Further, selective reflection of the incident light may partially occur.
In the liquid crystal display element, a predetermined voltage may be applied to the liquid crystal of the pixel for changing it from the focal conic state to the planar state. In this case, as shown in FIG. 6, the molecular orientation of liquid crystal of the pixel is controlled to attain the planar state. However, the liquid crystal between the neighboring pixels is affected by the applied voltage to attaint partially the planar state. Thus, the planar state and the focal conic state are mixed in the liquid crystal between the pixels.
For the above reasons, the planar state and the focal conic state are mixed in the liquid crystal between the pixels in the liquid crystal display element. In FIGS. 5 and 6, S indicates the substrate, T indicates the electrode, Lc indicates the liquid crystal molecules, P indicates the planar orientation state of the liquid crystal molecules, and F indicates the focal conic orientation state of the liquid crystal molecules.
As described above, a part of the incident light is selectively reflected and scattered by the liquid crystal between the pixels due to mixing of the focal conic state and the planar state of the liquid crystal between the pixels. This deteriorates the display characteristics of the liquid crystal display element.
According to the study by the inventors, if the rubbing processing is not effected on the substrate surface or the like for controlling the orientation directions of the liquid crystal molecules in the liquid crystal display element of the reflection type, the liquid crystal molecules between the substrates tend to be positioned in the random directions so that the view angle range allowing good observation of the display can be increased. This is already known.
However, if the rubbing processing is not effected for increasing the view angle, the liquid crystal molecules between the pixels are positioned in random directions. Therefore, the liquid crystal between the pixels forms small domains, and light scattering is liable to occur on the boundary between the domains.
As described above, in the liquid crystal display element or in the multilayer liquid crystal display element formed of the plurality of liquid crystal layers stacked together, the incident light may be scattered or selectively reflected (R1 in FIG. 7) if the light is applied to the liquid crystal between the pixels in each liquid crystal display element without effecting no control on the molecular orientation, as shown in FIG. 7.
In the multilayer liquid crystal display element A′, as shown in FIG. 7, the liquid crystal in the non-pixel region on the upper side (image observation side), i.e., the liquid crystal in the regions between the pixels scatters the light, which is selectively reflected by the liquid crystal display element lower than the liquid crystal display element nearest to the observation side, and passes toward the observation side (R2 in FIG. 7).
In this state, when performing the color display using the stacked liquid crystal display elements for display in red, green and blue, respectively, white display can be performed with high brightness owing to the selective reflection and scattering by the liquid crystal in the non-pixel domains. However, when performing, e.g., the black display by a light absorbing layer Bk in the focal conic state of the liquid crystal in the pixels, the black display is blurred due to the selective reflection and scattering of the incident light by the liquid crystal between the pixels, resulting in low contrast of the image display. Further, since the selective reflection and scattering of the incident light are caused by the liquid crystal between the pixels, and the liquid crystal between the pixels scatters the light, which is selectively reflected by the lower layer toward the observation side, these lower the color purity in display.
In any one of the above cases, the optimum solution has not yet achieved in connection with the orientation control of the liquid crystal in the above types of liquid crystal display element.